A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
A number of different patterns may be successively imaged at the same position on a semiconductor substrate. The substrate must undergo the desired physical and chemical changes between the successive images at the same position. To this end, the substrate must be removed from the apparatus after it has been exposed with a pattern, and, after it has undergone the desired process steps, the substrate must be replaced at the same position again so as to expose it with a second pattern, and so forth. Meanwhile, it must be ensured that the images of the second pattern and the subsequent patterns are positioned accurately with respect to the substrate. To this end, the lithographic projection apparatus is provided with an optical alignment system with which alignment marks on the substrate are aligned with respect to alignment marks on the patterning device (such as a mask).
In other words, before exposing the substrate, that substrate must be correctly aligned with respect to the rest of the system. An alignment mark is provided on the substrate and detected with an alignment system as discussed below.
A known alignment system is disclosed in WO 98/39689 and employs an alignment beam of radiation that is radiated by a separate alignment unit and that is incident on a mark, in the form of a grating, on the substrate. The grating diffracts the alignment beam into a number of sub-beams extending at different angles to the normal of the grating. Said distinct sub-beams will be directed with a lens of the alignment unit to different positions in a plane. In this plane, means may be provided for further separating the different sub-beams. The lens system will also be used finally to image the different sub-beams on a reference plate to create an image of the mark. In this reference plate, a reference mark can be provided and a radiation sensitive detector can be arranged behind the reference mark. The sub-beams may be diffracted by the reference mark to contain information about the alignment mark position with respect to the reference mark. The output signal of the detector will be dependent on the extent to which the image of the substrate mark and the reference mark coincide. In this way, the extent of alignment of the mark on the substrate with the reference mark in the alignment unit can be measured and optimized. The detector may comprise separate individual detectors for measuring the intensity and the aligned position at different orders. To finish the alignment, the reference in the alignment unit has to be aligned to a second reference mark, for example, one provided to the substrate table with the alignment unit. This second reference mark may then be aligned to a mark in the mask (or other patterning device) using exposure light.
The reference consists of a structure of a number of separate reference elements equal to the number of diffraction orders used and having the same shape as the substrate alignment mark. A separate detector is associated with each of these elements for converting the sub-beam coming from the substrate mark and passed by the relevant diffractive reference element into an electric signal. The structure of deflection elements comprises a pair of deflection elements for each diffraction order to deflect the sub-beams of this diffraction order with opposed diffraction order signs such that the second lens system converges these sub-beams on one associated reference element.
The deflection elements in the known alignment system comprise beam deflectors in the form of wedges. Specifically, minus order and plus order diffracted sub-beams (caused by the diffraction of the alignment beam) must be combined before producing the alignment image to be compared with the reference mark. In order that the plus- and minus order sub-beams of the same diffraction order are deflected such that they can be correctly superposed by the second lens system on the associated reference grating, stringent requirements are to be set to the mutual quality of the two associated wedges. These quality requirements relate to the quality of the inclined faces of the wedges and to the wedge angles. Severe requirements are necessary for the wedges to be set to the mutual accuracy of, for example, the slope of the wedge surfaces of two wedges which are used for deflecting the +order and −order sub-beams of the same diffraction order.
Furthermore, a large number of individual wedges are required to combine the sub-beams. For example, three wedges may be used for each sub-beam to make the wedge angle preciseness more robust. A series of wedges is then required for each sub-beam and in both the X- and Y-directions.
Another disadvantage of wedges is that when using two wavelengths in the alignment beam, the number of wedges must double, which means that instead of 24 discrete wedges, 48 wedges must be used, for example, or that instead of 6 wedge-shaped plates, 12 of such plates must be used.